Low power mercury switch



Nov. 7, 1967 1.. w. COOK 3,351,726

LOW POWER MERCURY SWITCH Original Filed March'l9, 1964 Inventor= LeonardW. Cook b mm a. mwci His Attorney United States Patent 3,351,726 LOWPOWER MERCURY SWlTCH Leonard W. Cook, Warwick, R1,, assignor to GeneralElectric Company, a corporation oi New York Continuation of applicationSer. No. 353,658, Mar. 19, 1964. This application Jan. 20, 1966, Ser.No. 533,105 4 Claims. (Cl. 200-452) This application is a continuationof application Ser. No. 353,658, filed Mar. 19, 1964-, now abandoned,which is a continuation-in-part of application Ser. No. 113,177, filedMay 29, 1961, now abandoned.

This invention relates to a low-power mercury switch; more particularly,the invention relates to a metal-envelope mercury switch of lowresistance and high sensitivity.

In my patent US. 3,098,137, there is disclosed a glow switch consistingof a cylindrical metal housing into which an axially positioned terminalrod insulated from the housing extends a short distance. A bimetalelement mounted on the portion of the terminal rod extending into thecylinder is distorted upon heating so as to make contact with the sidesof the cylinder. The present invention is an outgrowth of the inventionof US. 3,098,137, and can utilize the metal cylinder and terminal rodconstruction of that invention. However, in addition to a number ofother modifications which can be made to provide novel articles andperformance capabilities in general, a quantity of mercury issubstituted for the bimetal element so that it is the orientation ormovement of the cylinder rather than the temperature alone which effectsoperation of the switch. A mercury switch constructed in this mannerpossesses a number of valuable properties as will be outlinedhereinafter.

One of the objects of the present invention is to provide a durable,metal-enclosed, mercury switch.

Another object of the present invention is to provide a mercury switchhaving a very small operating angle.

Another object of the invention is to provide a small, lightweightmercury switch.

Another object of the invention is to provide a durable, metal-enclosedswitch which is unaffected by soldering or welding as an attachingmeans.

A further object of the invention is to provide a mercury switch whichoperates reliably at very low operating potentials.

The above and other objects are accomplished in accordance with thefollowing specification taken in conjunction with the attached drawingin which FIG. 1 is a sectional view of a mercury switch constructed inaccordance with this invention;

FIG. 2 is a perspective view of the metal elements of the inventionprior to assembly;

FIG. 3 shows the construction of the cylindrical portion of the switchas viewed from the flanged end; and

FIG. 4 is an enlarged sectional view across the cylinder taken along theline 4-4- in FIG. 1 showing the shape assumed by the mercury in closedposition. Briefly stated, in accordance with the present invention inone of its broader aspects, the invention is directed to a mercuryswitch comprising a hollow gas-tight cylindrical metal housing, at leastone terminal rod extending through one end of the housing and beinginsulatingly sealed from the housing, and a quantity of mercury in thehousing positionable to bridge a gap between a terminal rod and thehousing and to open the gap according to the orientation of the cylinderin a mass-attracting field.

Referring to the drawing, one form of a switch provided by the presentinvention consists of a cylindrical housing ltl having a closed end 11and a flanged end 12. Because of its low cost and because it can betreated in accordance with the present invention to resist the corro-'ice sive action of mercury, cold rolled steel is a particularlysatisfactory material for constructing the housing 10 as well as othermetal parts of the switch used in contact with mercury. More expensivematerial such as stainless steel and surface coatings of refractorymetal carbides, nitrides and borides would also be satisfactory innumerous such constructions as will be discussed more fully below.

It is sometimes desirable that the interior, or a portion of theinterior of the housing It) be surface roughened as by sand blasting,acid etching, or by grooving as by the longitudinal grooves 13 spacedaround the wall as shown in FIGS. 1 and 3. Regarding such surfaceroughening it has been found to reduce friction or the impediment toflow or other movement between the solid surface in contact with mercuryand the mercury itself, while preserving low resistance electricalcontact. However, to be effective the dimensions of grooving or othersurface roughening must be substantially smaller than the dimensions ofthe mercury globule bearing on the roughened surface. The method offorming ultrasensitive switches which includes the steps of surfaceroughening by particle impingement or etching, and the products formedfrom such methods, including a pointed axial contact terminal, aredescribed and claimed in my copending application S.N. 229,527, filedOct. 10, 1962.

To continue with the description of the device of FIG. 1, there iswelded to the flanged portion of housing 10 an assembly composed of aflange ring 14- and axially positioned terminal rod 15 sealed by meansof an inorganic insulating material 16. Terminal rod 15 extends a shortdistance into the housing 10 where it may be brought into contact with apool of mercury 1'7 in accordance with the angle of tilt of the housingwith respect to a gravity or other field which attracts the mass of themercury 17.

For the variety of switch illustrated in FIGS. 1-4, the

preferred amount of mercury in the switch is that quantity which, as itforms into a sphere, would have a diameter slightly less than the insidediameter of the housing 10. Mercury naturally tends to assume a globularshape and it is desirable for some applications of the switch to reducethe quantity of mercury used to a minimum in order to hold to a minimumthe friction of the mercury surface with the switch elements incontacts. Other applications or mercury switches such as in thermostatsoperated by bimetal elements involves a contrasting use of largeramounts of mercury and also is described in my copendmg application S.N.229,527, referred to above, where the object is to increase the switchsensitivity, i.e., to reduce its operating angle. This is done in partby increasing the contained mercury; in part by reducing flow resistanceor friction at the side walls, and in part by using a pointed centercontact element. Where such high sensitivity and small operating angleis not essential, smaller quantities of mercury may be employed eventhough the operating angle of the switch is thus increased. Reliableswitches may be formed with smaller amounts of mercury because it is inpart the roughened surface on the interior of the housing, such as thegrooves 13- which reduces the friction of the mercury. It is also inpart the cleanliness of the mercury surface, and absence from thesurfaces contacted by the mercury of elements which tend to contaminateor amalgamate with the mercury. Since mercury tends to assume a globularshape, the bottom portions of the grooves 13 (or the valleys of anyother type of roughened surface) do not make contact with the mercurythereby decreasing the friction of mercury with interior surfaces. Sincethe cylindrical surface is much greater than the surface ofthe terminalrod 15, this decrease does not significantly affect the electricalresistance through the switch.

Pursuant to the present invention, a switch is provided containing ahalf gram or less of mercury, having an overall weight of less than 2.5grams, a durable steel shell, low internal resistance and greatreliability of operation. Such a switch has numerous applications aswill be pointed out hereinafter.

The inorganic insulating seal 16 is composed of glass or a ceramicmaterial. A performed disk with a central aperture for the terminal rod15 is a convenient form for the seal. The flange 14 is made quite thickin order to withstand stresses which might be established duringfabrication. Where a preform is used for the seal 16, the preform ismounted on the terminal rod 15 and positioned in the central aperture ofthe flange 14. The assembly is then heated in a jig to a temperatureabove the softening point of the seal 16 to effect a gas-tight sealwhich insulates and positions the terminal rod with respect to theflange 14.

The foregoing describes some of the elements, ingredients and assembliesof the switch and from this description it may have a superficialappearance of being quite similar to switches taught in the art.However, the novelty and distinctness from the devices taught in the artwill become additionally clear from the descriptions which follow of themethods by which they are prepared.

In essence, the improved method of the present invention is thought toreside in one of its broader aspects in the selective removal of thosematerials which are responsible for the higher surface contactresistance and contamination of mercury switches formed with metalcasings. Stated in another way, the improved method resides in theformation on the interior of a metal casing of an irregular surface, theirregularities of which are of dimensions capable of supporting a liquidmetal of high surface tension therebetween and out of contact with asubstantial portion (greater than 50%) of the total surface; and whereinthe upper portions of the irregularities of the surface are highlyconductive; are in intimate electrical contact with the surfacesubstrate, and are incapable of being dissolved by liquid metals broughtinto contact therewith.

While this describes in general terms the nature of the improved contactsurface, its nature will become somewhat clearer by a furtherdescription of the several methods by which such a surface may be formedpursuant to the present invention.

A first method involves the selective removal of the more highlyresistant portions of a composite surface to expose the more highlyconducting, or potentially more highly conducting components of amulti-component solid material, from which an irregular surface of thepresent invention may be formed. This selective exposure method can beillustrated by reference to the method disclosed in my copendingapplication, S.N. 229,527, referred to above. As is pointed out therein,extreme care must be exercised in the manufacture of the mercury switchof this invention if it is to have a small operating angle. The interiorwall of the cylindrical housing is roughened by sand blasting oralternatively ultrasonically cleaned in detergent to remove any oilspicked up in processing and it is then subjected to a further acidetching process. The acid etching may be achieved by dipping three timesin 50% nitric acid for about 10 seconds each dip, with a distilled waterrinse after each dip. The housing is then ultrasonically cleaned indetergent after which it is cleaned in a hydrogen furnace atapproximately 1,000 F. The assembly formed as the closure member 14,terminal rod 15, and seal 16 is ultrasonically cleaned in detergent,rinsed in distilled water, and oven dried.

A measured quantity of ultra pure mercury is then metered into thecylindrical housing 10 and immediately thereafter a closuremember-terminal rod assembly is placed in position on the flange 12. Thehousing 1% is then evacuated to about 10 micron pressure. The housing 10is then filled with pure hydrogen or a rare gas such as argon at about50 pounds absolute pressure and the closure member 13 is sealed to theflange 12 as by welding.

To understand the significance of the foregoing procedure in formingirregular surfaces contemplated by the present invention, one mustbriefly review the chemistry which takes place during the process.Chemically, a mild steel contains a small percentage of manganese,sulfur, phosphorous, and carbon and the remainder iron. The sandblasting may be useful in increasing the surface area of the metalsurface which is to undergo treatment or which has already undergonetreatment. The nitric acid which then is used to etch the surface makesa more rapid attack thereon. An important factor in the results producedby this etching is that although the iron is soluble in and dissolved bythe nitric acid, the manganese contained in the iron is not as readilydissolved, but is rather oxidized to a manganese oxide insoluble innitric acid. This is particularly likely to result where at least aportion of the manganese is initially present as the oxide. Theprobability of at least some of manganese being present as the oxide isquite high inasmuch as the manganese is included in the iron primarilyas a deoxidizing and desulfurizing agent.

As is pointed out in my copending application, S.N. 229,527, as apreliminary step of the treatment procedure given above, the interiorwall of the cylinder 10 was acid etched by contact with nitric acid. Acomparison based on carrying out this step with sulfuric, hydrochloricand nitric acids showed that of these three acids, the nitric givessuperior end product. This superior end product is due in part to theselective removal of the iron and the attendant exposure of themanganese oxide which remains at the etched surface due to itsinsolubility in the nitric acid.

Regarding the remainder of the process, it is also known that themanganese oxide is not reduced by the furnace hydrogenation treatment,but that it is in fact reducible by hydrogenation only with hydrogen gasat elevated pressure. Accordingly, it is only after the switch has beensealed by welding to contain a 50 pound absolute pressure of purehydrogen that the conditions are provided for the reduction of thecrests of manganese oxide which have been exposed through the selectiveleaching and removal of the surface layer of iron with the nitric acid.

The foregoing method is one for use of liquid leaching or etching agentsto selectively expose an irregular pattern of a material incorporatedwithin a metal matrix and to form thereby an irregular surface, thepeaks or crests of which are both insoluble in mercury and either are orcan be rendered highly conductive to a liquid metal brought into contacttherewith.

However, the method of the present invention is not limited to the useof liquid leaching or etching agents to accomplish the selectiveexposure of a mercury-insoluble highly conductive material. Thisselective exposure also may be accomplished by means of gas and thefollowing procedure illustrates this alternative method of the presentinvention.

By this method a mixture of carbon dioxide, carbon monoxide and nitrogenis employed containing, in general, a minor proportion of the carbonoxide. In a particular treatment of mild steel parts, the parts werefurnaced on a travelling belt in an atmosphere containing 4.8% CO 10.6%C0; and the balance nitrogen. The furnacing was carried out at atemperature of about 1,800 F. and the belt traveled at a rate to keepthe parts in the hot zone of the furnace for about 15 minutes. The partswere then cooled in this same atmosphere, but outside the hot zone for10 minutes. This was followed by 5 minutes additional cooling at thedischarge end of the furnace gas flow tube through which the belttraveled. The cooled parts were then washed in denatured ethanol for 10minutes before being air dried with a hot air blower. To avoidcontamination following furnacing and prior to assembly, the parts werestored in dust-free containers such as clean hot covered jars.

This example illustrates the selective removal of iron from themanganese oxide of the mild steel partially as the volatile ironcarbonyl. In addition, due to the solubility of iron carbonyl inalcohol, the foregoing procedure may also, in part, provide for theremoval of the iron carbonyl with a liquid leaching agent.

While hydrogen is the principal atmosphere on the interior of the switchresulting from the above-recited method of manufacture, othernonoxidizing atmospheres are satisfactory. The rare gases, andparticularly argon, and mixtures of such gases with hydrogen, areentirely satisfactory. The pressure of 20-30 pounds per square inchabsolute creates reducing conditions on the interior which promote longlife. However, it is obvious that satisfactory results for manyapplications could be obtained from wider pressure limits.

If desired, a protective outer coating may be applied as byelectroplating of copper or dipping in molten zinc. The switch of thisinvention lends itself very well to miniaturization. The actual size ofa production model has an interior cylinder diameter of less thanone-quarter inch and a length, excluding the terminal rod portion whichprotrudes, of less than one inch. Such a switch uses approximately ahalf-gram of mercury. This switch lasted for over 2,000,000 on offcycles with a 100- watt lamp on a 110 volt A.C. circuit. The minimumoperation angle of the switch is about 4 degrees, and it is responsiveto 1 /2 volts or less. The switch is very rugged and has applications inmissiles where a gravity-responsive switch is desired as well as moreprosiac applications such as automobile trunk lights, refrigeratorlights, etc.

It may be incorporated in a circuit by soldering or welding the switchstructure without damage to the device.

It is to be noted that the switch is symmetrical about the axis of thecylindrical housing and terminal rod 15. In applications where theswitch can rotate about this axis during use, this feature extends thelife of the switch since the contacting surfaces constantly changethereby preventing undue corrosive wear at any one point. Byincorporating in the switch that quantity of mercury whose sphericaldiameter is only slightly less than the interior diameter of thecylindrical housing, the mercury tends to roll into contact positionbetween the housing wall and terminal rod to assume the shapeillustrated in FIG. 4an action which appears to improve the contactbetween the mercury and the rod housing.

From the foregoing it will be appreciated that although the rougheningof a metal surface can improve the flowability of mercury in contacttherewith, the lowering of the flow resistance or friction may be dueprimarily to a mechanical phenomena inasmuch as the amountof surfacecontacted is reduced. However, this surface roughening by itself is notsufficient to be the source of an increase of more than one thousandfoldwhich has been found in the conductivity between the metal surface andtheliquid mercury in contact therewith for switches prepared by theimproved methods of this invention.

Alternative methods of forming highly conductive mercury insolubleirregular surfaces is by a deposit on ,a receiving surface of a layer ofa particulate material having the desired properties. For this purposegenerally, the refractory metals and compounds of refractory metalswhich have relatively low resistance and relatively low solubility inliquid metals may be employed. For examr ple, tungsten, tantalum,molybdenum and similar metals having melting points near or above 2,000C. are useful either by themselves or in the form of compounds whichhave high conductivity and low solubility in liquid N metals. Forexample, titanium forms a highly conductive nitride as well as a carbidewhich is insoluble in liquid mercury even at elevated temperatures.Carbides, borides and nitrides of refractory metals are known and may beemployed in themethod of the present invention or incorporated inarticles hereof where the appropriate properties of high conductivityand low solubility in liquid metals are both present. Methods are knownin the art for forming deposits of such materials on steel surfaces. Forexample, a metal powder may be dusted on a steel surface and thenpressed into contact with the surface and then bonded there employingpowder metallurgy techniques. The use of powders and powder metallurgytechniques is particularly useful where formation of an irregularsurface layer is desired as the powder itself becomes the irregularsurface when bonded to the metal substrate.

One procedure which effects both good bonding and good electricalcontact between powder and metal substrate is a pressing of thedeposited powder into contact with the surface followed by a heating toform a sinter bond therebetween.

While the foregoing procedures yield highly satisfactory switches, it iswithin the scope of the present invention to employ alternativematerials, steps, and treatments in preparing switches which emphasizethe generation of internal surfaces having very low resistance coupledwith very low friction. It must be emphasized that these two propertiesare normally antagonistic in that in this art one normally associateslow resistance with a wetting contact between the mercury and internalmetal container surfaces but such Wetting severely restricts the flow ofmercury along the internal surfaces.

The importance of mercury flowability or low mercury flow resistance inthe switch is that it is this property which makes the switch motionsensitive, that is, highly responsive to changes in its positionrelative to a field attracting force such as gravity. As indicatedabove, the switch of FIGS. 1-4 containing about one-half gram of mercuryoperates when axially displaced through a minimum angle from its onposition to its off position of only four degrees. However, for otherswitch configurations, some of which are illustratively discussed below,reliance is not placed solely on. angular displacement of a switch axisabout which the switch is otherwise symmetrical.

The modifications of the switch design as well as of switch fabricationmethods disclosed herein permit use in low power circuits such as thoseoperating at voltages and amperages of the order of less than 0.001,volt and 0.001 ampere respectively without sacrifice in the motion orposition sensitivity.

This is more than a thousandfold lower useful power level than the levelgiven for the switch described in the predecessor application of thiscontinuation-in-part.

For many applications, particularly those using low power sources, it ismost desirable that the switch operate at low resistance. The modifiedmild steel mercury switches as described herein have been made tooperate at a switch resistance of the order of 0.01 ohm and less.

While a low operating resistance may be due in part to the design of theswitch shown in FIGS. 1 and 4 in providing a high level of contactbetween the central contact rod 15 and the shell 10, it can also beprovided in switches of other design due to the improved nature of theelectrical contact possible at the surface of the mild steel elements.

Because of the improved characteristics of the switch of the presentinvention, numerous modifications of structure and function can beaccomplished.

A number of these modifications involve a change in the structure orlocation or content of the contact post such as 15 of FIG. 1.

In one modification, for example, the post may be placed eccentricallywith respect to the casing 10 so that it will not be contacted uniformlyby any movement of mercury within the switch. Where the amount ofmercury is relatively smaller and the extent of eccentricity or axialdisplacement of the post 15 is larger, a

rotation of the switch about its axis when the axis is horizontal to afield of gravity, will produce a single signal corresponding to the flowof electricity from the post 15 to the shell 10 for each revolution ofthe switch. In this modification the switch becomes a rotationalcounting device. For example, where this modified form of the switch ismounted on a rotating member, the number of rotations of the member maybe determined by means of the signals produced from the switch.

This type of counting application of the modified switch is useful atlower speed where the globule of mercury will not be broken up by theforce of the post coming into contact with it. For rotary countingapplications it is important that the globule maintain its integrity asotherwise spurious signals may be produced.

Where higher speeds are to be employed, modifications of the switch mayinclude the incorporation of a ceramic spacer element in the container10 so that a moving mercury globule can pass over two or more electricalcontacts embedded in the ceramic spacer. The method of attaching aspacer having sections designed to regulate mercury flow within a sealedmetal casing is disclosed in copending application, S.N. 104,939, filedApr. 24, 1961 and assigned to the same assignee as the subjectapplication. It is evident that in a switch such as that describedtherein, having a ceramic closure element similar to 16 of the subjectapplication, but where the ceramic closure has a diameter ofapproximately /2 inch, that a substantial number of electrical contactpins can be sealed into the ceramic window. Where the contact pins aredistributed at their internal extensions into various sections of thespace enclosed by the switch, it is possible to produce a great varietyof switch constructions adapted to be used in numerous specificapplications. It is because the assembly of ceramic closure element 16,contact post 15 and metal ring are formed as a separate assembly to belater welded to the vessel 10, that accurate spacing of the severalcontact posts can be accomplished to form a large variety of switchingelements.

One such application is in use of the switch as a distributor such as isemployed with internal combustion engines to supply the electricalenergy to the spark plugs. Another application for the switch element isas a liquid brush in distributing electrical energy to the segments ofan electric motor. At the present time, both of these electricaldistribution functions are performed by mechanical contact. In the caseof motors, this is done by a contact of carbon with a metal surface inair and in the case of internal combustion motors, by the contact ofmetal conductor tips also in air. Because of the need for the electricalswitching elements to be used in air, their life expectancy is not asgreat as that of the switching element described herein inasmuch as itcan be used in a sealed container having a protective atmossphere.

While it has been the practice heretofore to employ mercury switcheswhere the electrical contact is between a solid surface and a mercuryglobule only for applications where the amount of electrical energytransmitted through the switch is relatively small, a distinct advantageis made possible by the present invention is that it permits mercuryswitches which rely on liquid-solid contact for electrical transmissionto be used for switching much higher levels of electrical energy.Evidence of the improved capability of the switch of the presentinvention is given above wherein it is noted that a small switchcontaining 'but /2 gram of mercury was employed in switching on the100-watt bulb through 2,000,000 cycles. By increasing the manganesecontent of the steel from which the switch is formed so that a morehighly refractory conductor surface is in contact with the mercury, theload carrying and load switching capacity of the switches is increased.

Materials which are used as coatings or liners for the switches of thepresent invention may be described generally as ceramic materials asthis term is generally used meaning that the materials are useful attemperatures above about 1,000 C. and which have vaporizationtemperatures which are substantially higher.

The value of the higher vaporization temperature lies in the resistancewhich this imparts to the surface layer being affected by electricaldischarges generated when an electrical contact is made or broken,Because the refractory material may comprise only a very thin layer onthe electrical contact surfaces within a switch, the need for highconductivity in the refractory material is not as great as where theceramic materials are employed in depth. Also, particularly where thethickness of the ceramic surface layer need not be substantial, switchescan be formed pursuant to the present invention which employ ceramicmaterials which are more costly, without substantially increasing in thecost of the switch. For this same reason, more unfamiliar material maybe employed where they have the combination of properties which renderthem suitable for use pursuant to the present invention. In general, therefractory metals such as tungsten, molybdenum and tantalum are usefulas metals to form electrical contact surfaces for liquid metals withinmetal encased switches. However, several other metals which are notnormally considered refractory metals, but which may be combined withother elements to form layers having high melting points and highvaporization points may be employed. For example, carbides, nitrides,borides, silicides and sulfides of various sorts had been considered asrefractory materials and uses can be made of such metal compounds whichpossess the set of properties discussed above.

For example, a material which has a useful combination of properties issilicon carbide. This has been used in many refractory shapes and has anoutstandingly good combination of thermal and electrical conductivity.The properties, in addition to good heat shock resistance, strengths athigh temperatures and abrasion resistance. Pursuant to the presentinvention, a deposit of particular silicon carbides may be formed at thesurface of metal casing of a switch element and will provide both lowflow resistance to contained mercury as well as high conductivity fromthe mercury through the carbide layer to the metal surface.

Another group of materials which may form surface liners for metalswitch housings are the so-called cermet materials which are intimatemixtures of metals and nonmetals. Illustration of such a material is acermet containing aluminum oxide, Al O and chromium. Although somecerinets are formed with oxides such as that indicated above is morecommon to incorporate a carbide or nitride as in cemented tungstencarbide. The surface formed on the switch of FIG. 1 in the mannerdescribed above may be thought of as closely analogous to a cermet priorto the reduction of the manganese compound by high pressure hydrogen.

While particular embodiments of this invention have been shown anddescribed, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, it is intended that theappended claims cover all such changes and modifications as fall withinthe true spirit and scope of this invention.

I claim:

1. In a metal enclosed mercury switch the improvement which comprises alow resistance at an internal surface, said surface being formed as aninternal surface of a steel casing containing at least 2.0% manganese inthe steel thereof, and said surface having a higher manganese contentthan that of the steel due to the preferential depletion of iron fromsaid surface.

2. The switch of claim 1 in which the manganese of said iron depletedsurface is at least partially present as manganese oxide, and said metalenclosure contains hydrogen under pressure.

3. The switch of claim 1 wherein the switch contains less than one-halfgram of mercury, the internal resistance of said switch through saidmercury is less than 0.01 ohm and wherein the switch is operable in lowpower circuits at less than 0.001 volt and 0.001 ampere.

4. A mercury switch comprising a hollow gas tight metal housing,

a quantity of mercury in said housing said metal housing being providedwith a roughened interior surface,

said roughened surface being formed of particles deposited on and bondedto said metal housing,

said particles being chosen from the group consisting of refractorymetals and compounds of refractory metals which are of high conductivityand low solubility in mercury at least one terminal rod extendingthrough one end of said housing,

References Cited UNITED STATES PATENTS Warren 200-152 XR Bear et a1200-152 Harrison 200152 Bucklen et al 200-152 Engel et a1 200-152 ROBERTS. MASON, Primary Examiner.

ROBERT K. SCHAEFER, Examiner. 20 H. J. HOHAUSER, Assistant Examiner;

1. IN A METAL ENCLOSED MERCURY SWITCH THE IMPROVEMENT WHICH COMPRISES ALOW RESISTANCE AT AN INTERNAL SURFACE, SAID SURFACE BEING FORMED AS ANINTERNAL SURFACE OF A STEEL CASING CONTAINING AT LEAST 2.0% MANGANESE INTHE STEEL THEREOF, AND SAID SURFACE HAVING A HIGHER MANGANESE CONTENTTHAN THAT OF THE STEEL DUE TO THE PREFERENTIAL DEPLETION OF IRON FROMSAID SURFACE.